Method and device for pressure sensing

ABSTRACT

A method for performing distributed pressure sensing including the steps of, forming a grating in a birefringent fiber, measuring the birefringence distribution along the length of the birefringent fiber, and determining pressure present along the length of the fiber using the measured the birefringence distribution. The invention also relates to a corresponding sensing device.

FIELD OF THE INVENTION

The present invention relates to a method and device for performingpressure sensing; and in particular to a method and device forperforming distributed pressure sensing which uses birefringentfiber(s).

DESCRIPTION OF RELATED ART

Optical fiber sensors have been extensively developed for measuringhydrostatic pressure in harsh environments such as oil/gas pipeline anddownhole applications.

To date, fiber-optic pressure sensing techniques based on fiber Bragggrating and fiber birefringence are used.

A Fiber Bragg grating (FBG) is made of periodic modulation of refractiveindex in fibers and acts as a light reflector.

Fiber Bragg gratings (FBGs) diffract light over a certain frequencyrange to provide a spectrum; the spectrum has a bell-shape and thecentral frequency of the spectrum is referred to as fiber Bragg gratingfrequency ν_(Bragg). Fiber Bragg gratings (FBGs) have been extensivelyimplemented as point and/or quasi-distributed pressure sensor since theFBG frequency ν_(Bragg) is inversely proportional to externally inducedstrain ε,

$\begin{matrix}{{V_{Bragg} = \frac{c}{2{n(ɛ)}\Lambda}},} & (1)\end{matrix}$

wherein c is light velocity in vacuum, n is refractive index of thefiber and Λ is the periodicity of refractive index modulation in FBGs,referred to as physical pitch.

It is possible, for instance using a piston mechanism, to couplehydrostatic pressure to a longitudinal movement which is measured by theFBG. Alternatively, a FBGs can be secured onto a material having a largePoisson module so that the pressure change can elongate or compress thematerial, hence elongating or compressing the FBGs. This is a pointsensor since the gratings are cascaded in a discrete manner with aperiodicity over a length of optical fiber so that the ambient pressureis monitored at only positions where FBGs are present

Standard fiber means that the refractive index is identical over anydirection due to symmetry of the cylindrical structure of the fiber.Birefringent fibers are fibers in which the refractive index n_(x) ofthe fiber along a horizontal axis (x-axis) is slightly different to therefractive index n_(y) of the fiber along a vertical axis (y-axis). Thetwo axis (horizontal axis (x-axis) and vertical axis (y-axis)) of thefiber are referred to as two primary axis in the birefringent fiber,where horizontal and vertical means orthogonal directions.

Light is an electro-magnetic wave, so there are electric field and alsomagnetic field propagating together. Polarization of light is simply thedirection of the electric field oscillation. Two primary polarizationmodes means that the electric field of the light is aligned along the x-or y-axis of the birefringent fiber.

Sensing techniques based on Fiber birefringence usually comparepropagation properties through a birefringent fiber of two orthogonalprimary polarization modes. The two orthogonal polarization modesexperience differential phase shift after propagating through thebirefringent fibers due to the refractive index difference(n_(x)−n_(y)).

The fiber birefringence B is used as a key element for pressure sensingsince the fiber birefringence is inversely proportional to ambientpressure with a coefficient C_(Bi), given as:

$\begin{matrix}{{C_{Bi} = \frac{B}{P}},} & (2)\end{matrix}$

wherein B=Δn=n_(x)−n_(y) (n_(x) and n_(y) being effective refractiveindexes along two primary axes x, y which are orthogonal to each other),and Δn is the difference between the effective refractive indexes n_(x)and n_(y).

So, an optical path length, which is the product of the physical lengthL of the fiber and the effective refractive index n of the fiber, isdifferent between two primary axis (i.e. n_(x)·L and n_(y)·L aredifferent since the effective refractive indexes n_(x) and n_(y), alongtwo primary axes x, y are different) so that the light experiencesdifferent phase shift between x-axis and y-axis.

The relative optical phase between the two modes at the end of the fiberhas a linear dependence on fiber birefringence. Since fiberbirefringence changes with pressure, a variation in pressure can bemeasured by monitoring the relative optical phase between the two modes.

A fibre laser cavity based on birefringent fiber can be used as anotherimplementation of pressure monitoring. The optical waves in thebirefringent cavity propagate according to two primary polarisationmodes, which optical frequencies are determined by the refractive indexn_(x) and n_(y), respectively; one optical wave is polarized along oneof two primary axis and the other wave is polarized along the otherprimary axis. These two waves may interfere at an exit of the cavity,generating a beat signal at frequency which is equal to the frequencydifference between the frequencies of the two waves. Due to the fiberbirefringence sensitivity to the pressure, the frequency of the beatsignal shifts proportionally with respect to the pressure change.

However, any change in the beat signal frequency or in the phasedifference between two orthogonal modes is the product of overallintegration over the entire length of the birefringent fibers; this is apoint sensor.

To date, fiber-optic pressure sensing techniques based on fiber Bragggrating and fiber birefringence can provide a high measurementresolution, but their implementations are still restricted to pointsensing.

Brillouin scattering in fibers has shown a possibility to realize adistributed pressure sensing system (e.g. to measure pressuredistribution along flowlines). The central frequency of Brillouinscattering resonance in fibers, referred to as Brillouin shift of fiberis given as:

$\begin{matrix}{{v_{B} = {\frac{2{nV}_{a}}{c} \cdot v_{p}}},} & (3)\end{matrix}$

wherein V_(a) is the velocity of an acoustic wave in the fiber, ν_(P) isan incident pump frequency and c is light velocity in vacuum. Theeffective refractive index n of the fiber has a linear dependence onpressure. Thus the Brillouin frequency ν_(B) shifts with respect to thepressure so that the hydrostatic pressure distribution along a sensingfiber can be computed by measuring distributed Brillouin frequency overthe fiber.

However, the Brillouin frequency sensitivity to hydrostatic pressure isinherently poor, of the order of 0.074 MHz/bar, which makes it uselessfor most applications. For example, such Brillouin scattering measuringtechniques cannot be used for oil reservoir management where pressuresensitivity around 0.1 bar is required.

It is possible to add a coating on the fiber, which transforms radialstrain into longitudinal strain so that hydrostatic pressure iseventually converted into axial strain. This way the sensitivity of theBrillouin measurement is improved by a large factor (for example aslarge as nearly 5) resulting in the enhanced sensitivity up to 0.340MHz/bar. The design and the fabrication of such special coating areextremely difficult and costly.

Thus Brillouin scattering in fibers was used as a promising solution tomeasure pressure distribution along flowlines, but this scheme requiresa transducer (a specifically designed coating) to enhance the Brillouineffect sensitivity to pressure, making it economically not viable andindustrially not practical.

There is a need in the art for a high-resolution distributed pressuremeasurement device and method which does not require the use of anexpensive coating on a sensing fiber to transform radial strain intolongitudinal strain.

BRIEF SUMMARY OF THE INVENTION

According to the invention, these aims are achieved by means of a methodof performing distributed pressure sensing comprising the steps of,forming a grating in a birefringent fiber, measuring the birefringencedistribution along the length of the birefringent fiber, and determiningdistributed pressure present along the length of the fiber using themeasured birefringence distribution.

The method of the present invention enables distributed pressure sensingusing gratings and birefringence in fibers. Fiber birefringence issensitive to pressure variation without the need of any transducers andthe grating generated in birefringent fibers is used as a means ofperforming the distributed birefringence measurement.

This invention provides distributed pressure measurement withsignificantly enhanced pressure sensitivity, by a factor possibly >100,compared to the prior art method which uses classic Brillouinscattering, but without the need for a transducer (e.g. a specificallydesigned coating).

A grating is made of a periodic modulation of refractive index along anoptical waveguide such as an optical fiber; and can diffract light whichpropagates through the optical fiber when the optical frequency of thelight satisfies Bragg conditions given by Eq. (1). For instance, whenthe grating is implemented in an optical fiber the grating is referredto as fiber Bragg grating (FBG) and the FBG frequency so that a changein the effective refractive index of the optical fiber will lead to ashift in FBG frequency.

The spatial resolution of pressure measurements along a fiber can bealso improved using the method of the present invention, whilepreserving the measurement resolution and accuracy, compared to theprior art methods.

The step of forming a grating in a birefringent fiber may compriseforming a static grating in a birefringent fiber.

The step of forming a static grating in a birefringent fiber maycomprise providing a birefringent fiber with a plurality of regionswherein the refractive index of the fiber is permanently modulated.

The method may comprise the steps of, providing a pulsed probe signal,polarized along a primary axis, to the birefringent fiber, scanning thefrequency of the pulsed probe signal to measure a first distributedfrequency at which maximum scattering is taking place; changing thepolarization of the pulsed probe signal to the orthogonal state withrespect to the previous state; scanning the frequency of the pulsedprobe signal to the measure a second distributed frequency at whichmaximum scattering is taking place; measuring the difference between thefirst and second distributed frequencies to determine the birefringenceof the birefringent fiber; repeating the above steps one or more times,and detecting a change in pressure applied to the birefringent fiber bydetecting a change in the difference between the first and seconddistributed frequencies.

The method may comprise the steps of, providing in the birefringentfiber a first probe signal, polarized along a first primary axis,scanning the frequency of the first probe signal to the measure a firstfrequency at which maximum scattering is taking place, providing in thebirefringent fiber a second probe signal, polarized along a secondprimary axis which is orthogonal to the first primary axis, scanning thefrequency of the second probe signal to measure a second frequency atwhich maximum scattering is taking place; measuring the differencebetween the first and second frequencies to determine the birefringenceof the birefringent fiber, detecting a change in pressure applied to thebirefringent fiber by detecting a change in the difference between thefirst and second distributed frequencies.

The step of forming a grating in a birefringent fiber may compriseforming a dynamic grating in a birefringent fiber.

The dynamic grating may be formed by Brillouin scattering.

The step of forming a dynamic grating in a birefringent fiber maycomprise, counter propagating a first pump signal and a second pumpsignal in the birefringent fiber so that the first pump signal and asecond pump signal interact by stimulated Brillouin scattering withinthe birefringent fiber, wherein the difference in between thefrequencies of the first pump signal and a second pump signal is withinthe range of the Brillouin frequency shift±an offset, wherein the offsetis equal to the spectral width of the stimulated Brillouin scatteringwithin the birefringent fiber. The spectral width of stimulatedBrillouin scattering between a pump and probe signals when they are bothcontinuous waves is typically in the range of 30 MHz in standard opticalfibers.

The difference between the frequencies of the first and second pumpsignals is preferably equal to a Brillouin frequency shift of thebirefringent fiber.

The first pump signal and second pump signal may be polarized along afirst primary axis of polarization.

The first pump signal and second pump signal may be continuous. Thefrequency or phase of the first and second continuous pump signals maybe correlated to provide a localized dynamic grating.

The first pump signal and second pump signal may be pulsed.

The probe signal may be pulsed. The probe signal may be continuous.

The method may further comprise the step of providing a probe signal,polarized along a second primary axis which is orthogonal to the firstprimary axis, in the birefringent fiber to measure the frequency of theprobe signal at which maximum scattering is taking place.

The method may further comprising the step of scanning the frequency ofthe probe signal to detect shift in the frequency at which maximumscattering is taking place, wherein the shift in frequency isrepresentative of a pressure change.

The probe signal may be temporally compressed to achieve higher spatialresolution.

The step of forming a dynamic grating may comprise co-propagating afirst and second pump signal in the birefringent fiber, reflecting thefirst and second pump signal using a reflecting means to provide firstand second reflected signals, interacting the first and second pumpsignals with the first and second reflected signals to generate adynamic Brillouin grating along the birefringent fiber, wherein thedifference in between the frequencies of a first pump signal and asecond pump signal is within the range of the Brillouin frequencyshift±an offset, wherein the offset is equal to the spectral width ofthe stimulated Brillouin scattering within the birefringent fiber.

Preferably the difference between the frequencies of the first andsecond pump signals is equal to a Brillouin frequency shift of thebirefringent fiber.

The method may further comprise the steps of performing distributedtemperature and/or strain measurements using the birefringent fiber; andsubtracting the distributed temperature and/or strain measurements fromthe distributed pressure measurement.

The method may comprise the steps of providing a birefringent fiberwhich is configured to be insensitive to ambient temperaturefluctuations, so the thermal cross sensitivity can be mitigated, andwherein the fiber is further configured such that the strain sensitivityof the birefringent fiber is mitigated.

The method may further comprise the step of modifying the birefringentfiber so that it is insensitive to ambient temperature fluctuations, sothe thermal cross sensitivity can be mitigated. For example thebirefringent fibers can be specially designed so that the thermalresponse along each axis is identical mitigating thermal crosssensitivity.

The method may further comprise the step of providing a birefringentfiber which is configured such that the strain sensitivity of thebirefringent fiber is mitigated.

According to a further aspect of the present invention there is provideda sensing device for performing distributed pressure sensing comprising,a birefringent fiber; a means for forming a grating in the birefringentfiber; a means for measuring birefringence distribution along the lengthof the birefringent fiber, and a means for determining distributedpressure present along the length of the birefringent fiber using themeasured birefringence distribution.

The birefringent fiber may be chemically doped so that it has improvedsensitivity to pressure. For instance, birefringent fibers can bespecially designed, so that the refractive index in one n_(x) of twoprimary axes is immune to the pressure while that the refractive indexn_(y) along the other primary axis is sensitive to the pressure, or viceversa so that the birefringence sensitivity to the pressure can beenhanced.

The birefringent fiber may be chemically doped with for example Boron.The structural properties of the birefringent fiber may be configured sothat the birefringent fiber has improved sensitivity to pressure; forexample the birefringent fiber may be configured to be a microstructured fiber. The birefringent fiber may be configured to comprisecavities; the cavities may be longitudinal arranged along the length ofthe birefringent fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood with the aid of the descriptionof an embodiment given by way of example only, and illustrated by thefigures, in which:

FIG. 1 provides a graphical illustration of a system according to afirst embodiment of the present invention suitable for theimplementation of a method according to a first embodiment of thepresent invention;

FIG. 2 provides a graphical illustration of a system according to asecond embodiment of the present invention suitable for theimplementation of a method according to a second embodiment of thepresent invention;

FIG. 3 provides a graphical illustration of a system according to athird embodiment of the present invention suitable for theimplementation of a method according to a third embodiment of thepresent invention;

DETAILED DESCRIPTION OF POSSIBLE EMBODIMENTS OF THE INVENTION

FIG. 1 illustrates a system 1 according to a first embodiment of thepresent invention, for performing distributed pressure sensing accordingto a method according to a first embodiment of the present invention.The system 1 comprises a birefringent fiber 3 in which there is written(i.e. generated) a static grating 5. The grating 5 can scatter lightwithin certain spectral bands 21, 22 and the scattered light withmaximum efficiency will have one of two different frequencies ν_(x),ν_(y); maximum scattering of light will occur at these frequenciesν_(x), ν_(y).

The birefringent sensing fiber 3 has a first refractive index n_(x)along a horizontal axis (x-axis) and a second refractive index n_(y)along a vertical axis (y-axis). The two axes (horizontal axis (x-axis)and vertical axis (y-axis)) of the birefringent fiber 3 are referred toas two primary axes in the birefringent fiber 3. Light which isconfigured such that its electrical field is aligned along the x-axis ispolarized along the x-axis and light which is configured such that itselectrical field is aligned along the y-axis is polarised along they-axis. The frequency of the light which is scattered by the staticgrating 5 will have two distinct frequencies ν_(x), ν_(y) depending onthe polarization of the light incident on the static grating 5.

The principle of operation of the system 1 relies on two features offiber birefringence:

Firstly, the frequency difference between the two distinct frequenciesν_(x), ν_(y) is representative of the birefringence of the birefringentfiber 3.

Secondly, the birefringence of the birefringent fiber 3 is sensitive topressure, so that a change in birefringence of the birefringent fiber 3will indicate a change in pressure. This is due to the fact that theeffective refractive indexes n_(x) and n_(y) of birefringent fiber 3differ according to pressure.

Pressure applied to the birefringent fiber 3 causes the two distinctfrequencies ν_(x), ν_(y) to shift by a different amount, so that thedifference between the two distinct frequencies ν_(x), ν_(y) changes. Achange in the difference between the two distinct frequencies ν_(x),ν_(y) can thus be used to compute a change in the pressure which isapplied to the birefringent fiber 3, according to the followingequation:

$\begin{matrix}{{\frac{\left( {v_{x} - v_{y}} \right)}{P} = {\frac{c}{2n\; \Lambda}\frac{\left( {\Delta \; n} \right)}{P}}},} & (4)\end{matrix}$

wherein d(Δn)/dP is a linear coefficient depending on fibers.

To measure shifts in the two distinct frequencies ν_(x), ν_(y)distributed measurement of birefringence along the entire length of thebirefringent fiber 3 must be taken.

To realize distributed measurement of birefringence along the entirelength of the birefringent fiber 3, two separate measurements ofdistributed scattering must be taken; specifically, a first measurementof distributed scattering along x-axis and a second measurement ofdistributed scattering along the y-axis, must be taken.

The distribution of the scattering can be obtained by launching a firstpulsed signal 12 a, which is polarized along the x-axis, into thebirefringent fiber 3; scanning the frequency of the first pulsed signalto find the frequency ν_(x) at which the maximum scattering is takingplace. Following this a second pulsed signal 12 b, which is polarizedalong the y-axis, is launched into the birefringent fiber 3; thefrequency of the second pulsed signal is scanned to find the frequencyν_(y) at which the maximum scattering is taking place. The scanning ofthe first and second pulsed signals 12 a,b ensure that the frequenciesν_(y), ν_(x) are distributed. Each of the first and second pulsedsignals 12 a,b are polarized by a linear polarizing means (not shown)before they are launched in to the birefringent fiber 3.

Once the distributed frequencies ν_(x), ν_(y) are determined, they aresubtracted to obtain the difference between the distributed frequenciesν_(x), ν_(y); the difference between the distributed frequencies ν_(x),ν_(y) reflects the distribution birefringence of birefringent fiber 3.Since the birefringence of the birefringent fiber 3 is proportional topressure, the measured magnitude of birefringence indicates themagnitude of pressure distributed along the birefringent fiber 3.

Any type of grating could be implemented in the birefringent fiber 3. Todate, several different techniques have been developed to creategratings in fibers, from static fiber Bragg gratings to dynamicgratings. Static long fiber Bragg grating can be considered as a moststraightforward solution to implement gratings along a fiber or aportion of the fiber, since the grating can be simply written in thefiber by illuminating an interference pattern of an intense light to thefiber. In the system 1 shown in FIG. 1 the grating 5 provided in thebirefringent fiber 3 is a static long fiber Bragg grating, wherein thegrating 5 is permanently written along a portion or the entire length ofa birefringent fiber 3 by means well known in the art.

Besides static fiber Bragg gratings which are permanently implementedalong a birefringent fiber 3, dynamic gratings could be also generatedin the birefringent fiber 3 of the sensing system 1.

Dynamic gratings are gratings which are temporally generated usingoptical interactions between two or more optical waves through a wideset of physical mechanisms such as stimulated Brillouin scattering,nonlinear Kerr effects, gain modulation, synthesis of optical coherencefunction and any other optical interactions or physical phenomena thatcan induce a periodic modulation of the effective refractive index alongthe fiber. Thus in the absence of optical interaction the dynamicgrating in the fiber vanishes. The optical properties of the dynamicgrating can be controllable by changing the optical properties of theoptical waves involved in the optical interaction. For this reason, suchgrating is referred to as dynamic grating.

FIG. 2 illustrates a sensing system 300 according to a second embodimentof the present invention suitable for the implementation of a methodaccording to a second embodiment of the present invention. In the system300 a dynamic Brillouin grating 31 is implemented in a birefringentfiber 33.

The dynamic Brillouin grating 31 is implemented using the followingsteps: A first pump signal 35 and a second pump signal 36 are counterpropagated (i.e. propagate in opposite directions) along thebirefringent fiber 33.

The first pump signal 35 is configured to have a frequency ν_(P1) andthe second pump signal 36 is configured to have a frequency ν_(P2) Thedifference between the frequencies of the first and second pump signalsν_(P1), ν_(P2) is preferably equal to the Brillouin frequency shift ofthe birefringent fiber 33 i.e. ν_(B)=ν_(P2)−ν_(P1). In such conditions,the refractive index in the fiber is periodically modulated due toelectrostriction phenomenon through the SBS process between the two pumpsignals so that a grating as long as the length of the birefringentfiber is generated in the fiber by the SBS process. Therefore thisgrating is referred to as dynamic Brillouin grating. The grating 31 canscatter light within certain spectral bands 41, 42 and the scatteredlight with maximum efficiency will have one of two different frequenciesν_(P2), ν_(S); maximum scattering of light will occur at thesefrequencies ν_(P2), ν_(S).

The first and second pump signals 35, 36 are linearly polarized alongthe x-axis before they are launched into the birefringent fiber 33. Thefirst and second pump signals 35, 36 can be linearly polarized by meansof a linear polarizer.

It will be understood that the difference between the frequencies of thefirst and second pump signals 35, 36 could be equal to the Brillouinfrequency shift of the birefringent fiber 33 or anywhere within therange of the Brillouin frequency shift of the birefringent fiber+−thespectral width of the stimulated Brillouin scattering within thebirefringent fiber 33.

The first and second pump signals 35, 36 will interact to result inStimulated Brillouin Scattering (SBS). The optical interaction of thefirst and second pump signals 35, 36 during SBS result in the generationof the dynamic Brillouin grating 31.

Next a pulsed probe signal 39 is provided in the fiber 33. The pulsedprobe signal 39 is polarized along the y-axis (i.e. orthogonal to thex-axis along which the first and second pulsed signals 35, 36 arepolarized). The probe signal 39 is configured to have a frequency whichsatisfies the following condition:

$\begin{matrix}{{v_{B} = {{\frac{2V_{a}}{c}n_{x}v_{P\; 2}^{x}} = {\frac{2V_{a}}{c}n_{y}v_{S}^{y}}}},} & (5)\end{matrix}$

wherein n_(x) and n_(y) are the effective refractive indexes of thebirefringent fiber 33 along the x and y axes respectively; ν^(x, y)represents the state of polarization of the first and second pumpsignals 35, 36 and V_(a) is sound velocity in the birefringent fiber 33.When the probe signal 39 satisfies this condition then the dynamicBrillouin grating 31 will scatter the probe signal 39.

The frequency of the probe signal 39 is scanned to measure thescattering along length of the birefringent fiber 33. The frequency ofthe probe signal 39 at which the maximum scattering is taking place(i.e. frequency (ν_(s))) is identified.

The frequency of the probe signal ν_(s) at which the maximum scatteringis taking place is determined, according to:

$\begin{matrix}{{v_{S} = {\left( {1 + \frac{\Delta \; n}{n}} \right)v_{{P\; 2}\;}}},} & (6)\end{matrix}$

wherein Δn is the fiber birefringence, n is refractive index of thefiber and ν_(P2) is the frequency of the second pump signal.

Since the frequency of the second pump signal is fixed during operationof sensing, it can be clearly seen in Equation (6) that the frequencydifference Δν (Δν=ν_(s)−ν_(P2)) between the second pump signal 36frequency ν_(P2) and the frequency at which maximum scattering is takingplace ν_(g), has a linear dependence on the fiber birefringence Δn. Sochange in the frequency difference Δν between the frequency of the probesignal at which the maximum scattering is taking place ν_(s) and thefrequency of the second pump signal ν_(P2), will reflect a change in thebirefringence (delta n) of the fiber 3, which in turn reflects a changein pressure, according to:

$\begin{matrix}{\frac{\left( {\Delta \; v} \right)}{P} \approx {\frac{v_{P\; 2}}{n} \cdot {\frac{\left( {\Delta \; n} \right)}{P}.}}} & (7)\end{matrix}$

Typically the birefringent fiber 33 is made by modifying the refractiveindex of a core and cladding of the birefringent fiber 33. In general,the fiber birefringence of birefringent fiber 33 may vary with respectto any other physical quantities such as temperature and strain. So, anypossible cross sensitivity would preferably be discriminated tounambiguously determine the amount of pressure change. For instance, thebirefringent fiber 33 can be modified to be insensitive to ambienttemperature fluctuation, so the thermal cross sensitivity can bemitigated. The birefringent fiber 33 comprises a core and cladding (notshown); doping the core and/or cladding with certain chemicals (e.g.Boron) will result in a mitigation of the thermal cross sensitivity.Alternatively the birefringent fiber 33 may comprise photonic crystalfibers so that the thermal and strain sensitivity of the fiberbirefringence can be mitigated. In addition, temperature/strainresponses can be also completely discriminated by performing additionaldistributed temperature/strain measurements through the samebirefringent fiber 33; the distributed temperature/strain measurement isthen subtracted from the distributed pressure measurement. Furthermore,the birefringent fiber 33 can be configured to have enhancedbirefringence sensitivity to the pressure, for example the birefringentfiber 33 may be configured to be a micro structured fiber. An enhancedbirefringence sensitivity will further improve measurement resolution.

FIG. 3 illustrates a sensing system 500 according to a third embodimentof the present invention which is configured to implement a thirdembodiment of method according to the present invention. The sensingsystem 500 shown in FIG. 3 has many of the same features of the sensingsystem 300 in FIG. 2 and like features are awarded the same referencenumerals.

In sensing system 500, the first and second pump signals 35, 36co-propagate (i.e. propagate in the same direction) along thebirefringent fiber 33. The first and second pump signals 35, 36 are thenreflected by a light reflector 50, such as a standard mirror or Faradayrotation mirror, placed at an end 51 of the birefringent fiber 33, toprovide first and second reflected signals 53, 55. The first and secondpump signals 35, 36 interact with the first and second reflected signals53, 55 within the birefringent fiber 33 to generate dynamic Brillouingratings 31 within the birefringent fiber 33.

Following the generation of the dynamic Brillouin gratings 31, thedistributed pressure along the birefringent fiber 33 is determined usingthe same steps as outlined above with respect to FIG. 2.

Various modifications and variations to the described embodiments of theinvention will be apparent to those skilled in the art without departingfrom the scope of the invention as defined in the appended claims.Although the invention has been described in connection with specificpreferred embodiments, it should be understood that the invention asclaimed should not be unduly limited to such specific embodiment.

1. A method of performing distributed pressure sensing comprising thesteps of, forming a grating in a birefringent fiber, wherein the gratingis a continuous grating; measuring the birefringence distribution alongthe length of the birefringent fiber, and determining distributedpressure present along the length of the fiber using the measuredbirefringence distribution.
 2. The method according to claim 1 whereinthe step of forming a grating in a birefringent fiber comprise forming astatic grating in a birefringent fiber.
 3. The method according to claim2 wherein the step of forming a static grating in a birefringent fibercomprises providing a birefringent fiber with a plurality of regionswherein the refractive index of the fiber is permanently modulated. 4.The method according to claim 2 comprising the steps of, providing apulsed probe signal, polarized along a primary axis, to the birefringentfiber, scanning the frequency of the pulsed probe signal to the measurea first distributed frequency at which maximum scattering is takingplace; changing the polarization of the pulsed probe signal, scanningthe frequency of the pulsed probe signal to the measure a seconddistributed frequency at which maximum scattering is taking place,measuring the difference between the first and second distributedfrequencies to determine the birefringence of the birefringent fiber,repeating the above steps one or more times, and detecting a change inpressure applied to the birefringent fiber by detecting a change in thedifference between the first and second distributed frequencies.
 5. Themethod according to claim 1 wherein the step of forming a grating in abirefringent fiber comprises forming a dynamic grating in a birefringentfiber.
 6. The method according to claim 5 wherein the dynamic grating isformed by Brillouin scattering.
 7. The method according to claim 5,wherein the step of forming a dynamic grating in a birefringent fibercomprises counter propagating a first pump signal and a second pumpsignal in the birefringent fiber so that the first pump signal and asecond pump signal interact by stimulated Brillouin scattering withinthe birefringent fiber, wherein the difference in between thefrequencies of the first pump signal and a second pump signal is withinthe range of the Brillouin frequency shift±an offset, wherein the offsetis equal to the spectral width of the stimulated Brillouin scatteringwithin the birefringent fiber.
 8. The method according to claim 7wherein the difference between the frequencies of the first and secondpump signals is equal to a Brillouin frequency shift of the birefringentfiber.
 9. The method according to claim 7 wherein the first pump signaland second pump signal are polarized along a first primary axis ofpolarization.
 10. The method according to claim 9 further comprising thestep of providing a probe signal, polarized along a second primary axiswhich is orthogonal to the first primary axis, in the birefringent fiberto the measure the frequency at which maximum scattering is takingplace.
 11. The method according to claim 10 further comprising the stepof scanning the frequency of the probe signal to detect shift in thefrequency at which maximum scattering is taking place, wherein the shiftin frequency is representative of a pressure change.
 12. A methodaccording to claim 5 wherein the step of forming a dynamic gratingcomprises co-propagating a first and second pump signal in thebirefringent fiber, reflecting the first and second pump signal using areflecting means to provide first and second reflected signals,interacting the first and second pump signals with the first and secondreflected signals to generate a dynamic Brillouin grating along thebirefringent fiber, wherein the difference in between the frequencies ofthe first pump signal and a second pump signal is within the range ofthe Brillouin frequency shift±an offset, wherein the offset is equal tothe spectral width of the stimulated Brillouin scattering within thebirefringent fiber.
 13. The method according to claim 1 furthercomprising the step of performing distributed temperature and/or strainmeasurements using the birefringent fiber subtracting the distributedtemperature and/or strain measurements from the distributed pressuremeasurement.
 14. The method according to claim 1 further comprisingproviding a birefringent fiber which is configured to be insensitive toambient temperature fluctuations, so the thermal cross sensitivity canbe mitigated, and wherein the fiber is further configured such that thestrain sensitivity of the birefringent fiber is mitigated.
 15. A sensingdevice for performing distributed pressure sensing comprising, abirefringent fiber; a means for forming a grating in the birefringentfiber wherein the grating is a continuous grating; a means for measuringbirefringence distribution along the length of the birefringent fiber,and a means for determining distributed pressure present along thelength of the birefringent fiber using the measured birefringencedistribution.